v2 Cryosphere Research

How will the atmosphere, ocean and sea-ice systems mediate sources of sea-level rise from the Antarctic ice sheet over the next 30 years?

Global Mean Sea-Level (GMSL) rise remains one of the most impactful and uncertain consequences of a warming climate. While the Fourth National Climate Assessment Special Science Report adopted projections of GMSL rise by 2100 in the range of 30 cm to 120 cm, the possibility of GMSL rise of ~240 cm could not be excluded (USGCRP, 2017). As the costs associated with sea-level rise increase nonlinearly with GMSL (Boettle et al., 2016), the probabilities of future GMSL at the high-end tails carry outsized economic impacts. E3SM is uniquely positioned to better constrain the high-end estimates of future GMSL, which is dominated by uncertainties associated with rapid, unstable collapse of the West Antarctic ice sheet (the “marine sheet instability”; Schoof, 2007). Constraining projections of GMSL requires progress along two fronts: (1) an accurate representation of the relevant ice-sheet-climate forcings, and (2) a robust coupling of dynamic ice sheet models to those forcings within the coupled Earth system. While the latter is being addressed through the SciDAC ProSPect project, the former will be a focus of the project’s v2 cryosphere science campaign. Specifically, the focus will be on evaluating and improving the representation of the climate forcings that can rapidly melt ice shelves from below (the ocean) and above (the atmosphere), thereby increasing the likelihood of unstable ice sheet retreat. This goal is distinct from the ProSPect project, which focuses mainly on developing a dynamic ice sheet model and its coupling to other Earth system components.

Theory, observations, and modeling all support the hypothesis that rapid and unstable retreat of the majority of the West Antarctic Ice Sheet, and parts of the East Antarctic Ice Sheet, can occur following the collapse of its fringing ice shelves (Schoof, 2007; Scambos et al., 2000; Cornford et al., 2016). Collapse of the ice sheet would emerge due to a set of complex, coupled physical processes involving the atmosphere, ocean, and sea ice systems. One of the largest sources of uncertainty with respect to abrupt GMSL rise is from sub-marine melting of ice shelves associated with relatively warm ocean water intrusions onto the Antarctic continental shelf (USGCRP, 2017). Melting at the ocean-land ice interface depends on the efficiency of the ocean-ice heat exchange: how much heat can be brought into the ice shelf cavities, and how much heat can be extracted from the ocean to melt ice within the ice shelf cavities? The former depends strongly on the structure and position of water masses in the Southern Ocean, particularly the location and outcropping of the relatively warm Upper Circumpolar Deep Water (UCDW). The latter depends on both the structure and evolution of the near-coast currents, such as the Antarctic Slope Front Current (Stewart and Thompson, 2013; Stewart and Thompson, 2015) and Antarctic Coastal Current (Spence et al., 2014). In addition, the flux of freshwater at the ocean-land ice interface is extremely sensitive to the strength and structure of the sub-ice ocean circulation, as well as the areal extent of the land ice interface.

Atmospheric winds in the Southern Hemisphere play a leading role in determining the outcropping location of UCDW and, more broadly, in setting the structure of Southern Ocean water masses (Hallberg and Gnanadesikan, 2006). At hemispheric spatial scales, biases in the surface westerlies drive anomalous ocean water upwelling leading to incorrect outcropping of UCDW. At much finer spatial scales typically not resolved in ESMs, persistent katabatic winds drive the formation of coastal polynyas which, in turn, produce Antarctic Bottom Water (AABW; Tamura et al., 2008) that, in large part, sets the global ocean circulation and mediates the flow of warmer waters onto the continental shelf. There is strong evidence that the Antarctic ozone hole has increased wind speeds at the ocean surface around Antarctica (Solomon et al. 2015; Thompson et al. 2011), which will presumably then affect sea-ice concentration and distribution and upwelling of warm intermediate depth waters that impact ice-shelf melting. The atmosphere also impacts ice shelf integrity via the “surface mass balance” (SMB). For Antarctica, SMB is currently positive almost everywhere throughout the year (van de Berg et al., 2006). However, warm air intrusions can generate substantial surface melt on flat, low lying ice shelves, as documented by the ARM West Antarctic Radiation Experiment (AWARE) in January 2016. Surface melt ponding in small fractures and larger crevasses can lead to the process of “hydrofracture,” whereby water-filled crevasses penetrate the full-thickness of ice shelves, eventually leading to their collapse (Scambos et al., 2000; Banwell et al., 2013), such as the collapse of the Larsen B ice shelf in 2002 (Glasser and Scambos, 2008).

Like the atmosphere, sea ice processes strongly modify the structure of water masses in the Southern Ocean (Abernathey et al., 2016) and mediate ocean-land ice melt. Water mass modification occurs directly through the redistribution of freshwater and indirectly by shielding the ocean from the atmospheric surface winds. Southern hemisphere sea ice is almost entirely seasonal and this seasonality results in a net poleward transport of ocean heat. In addition, during the seasonal, sea-ice life cycle, the ice intercepts atmospheric precipitation that also tends, on average, to move freshwater equatorward. In terms of impacting ocean-land ice melting, sea ice also plays a critical role of attenuating or even completely shielding the ocean from atmospheric wind stress in and near coastal regions (Dinniman et al., 2011). Slight changes in sea-ice coverage lead to substantial changes in the amount wind stress applied to the ocean surface that, in turn, can determine whether UCDW enters sub-ice cavities and induces ice melt (Dinniman et al., 2012, Hellmer et al., 2012).

Given the challenges in modeling the ocean, atmosphere, and sea ice forcings that influence the Antarctic Ice Sheet, our v2 cryosphere systems simulation campaign will use a combination of coupled and uncoupled simulations to understand and reduce biases of the climate forcings for the ice sheet and perform numerical experiments to delineate how the atmosphere, ocean, and sea ice mediate the contributions of the Antarctic Ice Sheet to sea-level rise in the near future.